The field of the invention comprises stretch gaskets for oven doors and the background and advantages represented by the stretch gasket generally over prior gaskets used for oven doors are laid out fairly well in the incorporated patents.
The problem which is solved by the invention herein is one which has arisen for several reasons and it is concerned with the bond which is effected between the metal insert of the gasket and the tubular interior thereof.
The efficacy of the basic concept of cementing a metal insert into an end of a hollow silicone rubber gasket is beyond argument inasmuch as such gaskets, commonly called stretch gaskets, are in universal use at the present time. Where conditions are normal an excellent bond is produced by the usual method of installation and the ordinary type of insert. The usual method of installation consists of injecting a quantity of RTV cement into the hollow gasket ends and or dipping the insert into RTV and thereafter pushing the insert into the gasket end and setting the gasket aside to self-vulcanize.
As a result of variations in dimentions, silicone rubber composition, ambient termperature and humidity conditions, and the technique of the worker, the bond which is achieved may be of variable strength. The usual insert is about one quarter inch wide and of sheet steel about 1/32 inch thick and has about one and three quarters of an inch engaged within the end of the gasket. A properly installed insert will have a pull-out strength of 12 to 15 pounds. Even lesser pull-out strengths are usable without difficulty, but the gasket installers at the oven factory and the householders exert different degrees of pull on the gaskets during their installation and removal from the oven face. When the pull-out strength drops substantially below the values given above, the possibility of separating the RTV bond of the steel insert from the silicone rubber gasket interior increases.
One factor which it is felt may contribute materially to this is that the insert fits in the gasket end closely and the act of insertion during manufacture will wipe the adhesive from the surface of the insert providing little or no adhesive to establish the bond.
According to the invention, by a modification of the structure of the insert the establishment of an adhesive bond is ensured and the pull-out strength increased very substantially over the pull-out strength of prior stretch gaskets. The possibility of faulty stretch gaskets with low pull-out strength being made is materially decreased.
In the practice of the invention herein, the insert is provided with notches alongside of its edges, giving rise to wings or teeth between the notches but according to the invention herein, such teeth are rounded and not sharp or pointed in any way. The purpose thereof is not to dig or cut into the walls of the gasket body but to provide the separation between the notches and to fill out the space transversely of the tubular socket so that the insert is firmly placed in the gasket. It differs in this respect from the structure of U.S. Pat. No. 3,404,675 and one of the embodiments of U.S. Pat. No. 3,765,400 both of which disclose the use of barbs alongside the edges of the insert. These latter structures are not satisfactory because they tend to pierce the gasket and render the same useless.